Combined cycle systems are comprised of:
1) One or more gas turbines each driving an electric power generator;
2) A steam turbine train comprised of steam driven turning a common drive shaft that drives an electric power generator; and
3) A heat recovery unit in which heat in the combined gas turbine combustion exhaust gas stream is transferred to the steam turbine condensate to raise steam for the steam turbine train.
Optionally, a conventional steam boiler unit can be included in the complex to produce additional high pressure steam to increase power output from the steam turbine train. The condensate recycle system for the steam boiler and the heat recovery unit are integrated.
Modern combined cycle units fire fuel gas in the heat recovery unit to supplement the sensible heat transferred from the gas turbine exhaust gas streams to the steam turbine train working fluid. This feature is termed supplemental fuel gas firing. The additional heat released by supplemental fuel gas firing is used to increase steam generated in the heat recovery unit to increase power output from the steam turbine train.
Fuel gas for supplemental firing is injected directly into the gas turbine combustion exhaust gas stream in the heat recovery unit. The gas turbine exhaust gas streams contain sufficient residual unburned oxygen to support combustion of the fuel gas in the heat recovery unit.
The fuel to power efficiency of the incremental power generated by supplemental firing is less than for fuel gas fired to drive the gas turbines. Accordingly, supplemental firing is usually practiced to make incremental power during periods of peak power demand, when power prices are high. Also, supplemental firing is used when sensible heat available from the gas turbine exhaust gas stream is not adequate to raise all the steam required for the steam turbine train.
The fuel gas commonly used for supplemental firing is natural gas. Natural gas is the fuel of choice for gas turbines and therefore is already delivered on site.
Commercially available processes exist that convert a broad range of low cost solid hydrocarbon bearing materials to low BTU gas (100 to 150 BTU heating value per 1000 SCF) and mid BTU gas (300 to 400 BTU heating value per 1000 SCF heating value). Low and mid BTU gas is comprised of hydrogen, carbon monoxide, carbon dioxide, methane, nitrogen, and water. Materials that can be gasified to make low and mid BTU gas include bituminous and sub-bituminous coals, lignite, petroleum cokes, and bio-mass materials. Typical gasification processes for making fuel gas from coal are described in Chapter 6 of the Handbook of Refining Processes published by McGraw-Hill in 1996 (edited by Robert A. Meyers) and are incorporated herein by reference. This reference teaches using low and mid BTU gas to fire the gas turbines in combined cycle power generator units.
Many petroleum refineries include continuous fluidized-bed thermal cracking units. These units thermally crack heavy crude fractions such as vacuum residuum, atmospheric residuum, tar sands bitumen, heavy whole crude, shale oil, and catalytic plant bottoms to produce coke naphtha and gas oils. The cracking units are integrated with coal gasification reactors that produce low BTU gas (120 to 140 BTU) per standard cubic foot lower heating value) from coke. One such commercial process is the ExxonMobil Flexicoking process that is described in described in Chapter 12 of the aforementioned Handbook of Refining Processes and is incorporated herein by reference. The reference teaches burning the low BTU gas produced in the Flexicoker in process heaters and boilers.
Coal, lignite, and coke usually contain sulfur compounds in varying amounts. The sulfur is converted to hydrogen sulfide and carbonyl sulfide during the gasification process. Sulfur in the product gas is undesirable because it is converted to sulfur oxides when the gas is burned and sulfur oxides are of course unacceptable air pollutants. Accordingly, to reduce the sulfur content of the product low BTU gas, a sulfur adsorbing compound such as limestone or dolomite can be added to the gasification reactor to absorb the sulfur. Limestone and dolomite typically reduce the sulfur content of the product gas to 300 to 600 ppm sulfur by weight
Alternatively, the product low BTU gas can be treated to remove and recover residual sulfur and optionally also carbon dioxide in a suitable sulfur commercial recovery unit downstream of the gasification unit. The ExxonMobil Flexorb process, described in Chapter 12 of the aforementioned Handbook of Refining Processes and incorporated herein by reference, is a widely used commercially available sulfur adsorption process which can reduce the sulfur content of the product low BTU gas to below 10 ppm by weight.
Whereas is with conventional combined cycles the general practice is to fire natural gas in the heat recovery unit only during periods of high power demand to generate peak power when higher power prices can support the higher cost of supplemental power, the economics of the combined cycle of this invention favors continuous injection of low or mid BTU fuel gas produced with the gasifier unit to produce supplemental power continuously. This is because fuel gas produced with the gasifier is made from low cost materials with existing facilities and is therefore usually cheaper than natural gas. Also, it is cost effective and easier to operate a process unit such as the gasifier continuously rather than to start it up and shut it down on an erratic schedule.